my %messages = map { $_ => [] } qw(error alert warning);
my $logfile = '/var/log/messages';

open my $log, "<", $logfile or die "$logfile: open: $!";
while (<$log>) {
    for my $type (keys %messages) {
        push @{ $messages{$type} }, $_ if /$type/;
    }
}
close $log or die "$logfile: close: $!"; # as long as we're bothering with explicit close, we may as well test it for success.